I sincerely suggest you get her a gift card.
However, if you are short on time, or she won’t want to buy herself an evening dress for some reason, I will recommend buying a transformer. Like this:
She will be able to create about 10 different styles and gazillion of looks, and the only measurement you need is chest measurement and cup size. I think you can buy those dresses here on Dahlia Couture
Buying clothes without the person who’ll be wearing it with you is tricky, especially if the other person is a girl. You will need to know what size she normally wears and be aware of any body parts she may or may not want to emphasize. We can help with this consulting
In addition, your definition of sexy may be different from hers. Of course, you can take a peak in her wardrobe but she might be wondering what you were doing, thereby ruining the surprise.
Still, if you insist. Below are 3 different photos of black dresses we can custom to fit your girlfriend/wife/spouse or sister and we think it will look pretty sexy
Images from various websites
Instead, I would suggest either giving her a gift certificate to purchase from our online store (We discount for love) so she can buy the dress or you can come with her on her shopping trip so both of you can agree on the dress. At the end of the day, she’s the one who’s going to wear it so you have to make sure she likes the dress.